1. Field of the Invention
The present invention relates to a system for summing RF signals modulated on an optical carrier and more particularly, to a system which compensates for optical fading of the RF modulated optical signals at a summing junction in which the RF modulated optical signals are each applied to a four-wave mixing element to produce phase conjugates of the signals, which, in turn are summed at a summing junction.
2. Description of the Prior Art
In order to transmit RF signals with higher frequencies, such as microwave and millimeter wave frequencies, and wider signal bandwidths, optical communication systems are known to be used. In such systems, the RF signals are modulated onto an optical carrier. The RF modulated optical signals are then used in various optical communication systems. Examples of such optical communication systems are disclosed in U.S. Pat. Nos. 4,233,606; 4,767,195; 5,064,140 and 5,493,691.
In such optical communication systems, the RF modulated optical signals are transmitted through the atmosphere. In order to compensate for atmospheric perturbations which affect the performance of such optical communication systems, optical phase conjugation techniques are known to be used. Such optical phase conjugation techniques are known to compensate for atmospheric perturbations of the optical phase of the optical carrier.
Certain other applications of RF modulated optical signals are problematic, such as phased array antenna application. Phased array antenna systems are generally known in the art. Examples of such phased array antenna systems are disclosed in U.S. Pat. Nos. 3,611,400; 3,999,182; 4,079,379; 4,079,381; 4,123,759; 4,612,547; and 4,717,918. Such phased array antennas normally consist of a number of linearly arranged antenna elements that form an array. The antenna elements are normally coupled to a single RF modulated optical signal source. Since the antenna elements are fixed in position, the radiation elements can be altered or steered by controlling the electrical phase or time delay of the optical signal applied to each of the antenna elements. The phase or time delay control of the individual antenna elements allows the composite beam to be scanned over an area of interest without movement of an antenna.
When such phased array antenna systems use optical carriers for RF beam forming, they require the summing of RF modulated optical signals. Optical combining of RF modulated optical signals is complicated by optical fading at the summing junction, introduction of non-linearities for single mode couplers and the limited number of high frequency signals that can be combined at a single photodetector. Known solutions involve either the stringent control of the optical phase which is relatively expensive and impractical in most applications or the use of multiple optical wavelengths. The use of multiple optical wavelengths is also limited either by the frequency response of the applicable photodetector or the number of wavelengths required.